This invention pertains generally to methods for improving the performance of detectors for gamma-ray and x-ray spectrometers and imaging systems, and particularly to a method for treating the surface of CdZnTe detector crystals to reduce leakage currents and increase spectral resolution.
Many of the commonly used radiation detectors employ Si(Li) or Ge semiconductor materials and thus operate most effectively at cryogenic temperatures and in a very clean vacuum. The need to operate Si(Li) or Ge-based detectors under these rigorous conditions poses significant limitations on the use of these materials in those applications where portability is desired. The general requirement for room temperature operation of a semiconducting material as a nuclear detector and spectrometer is a relatively large band gap energy such that thermal generation of charge carriers is kept to a minimum. Conversely, the requirement for high resolution is a small band gap energy such that a large number of electron-hole pairs is created for an absorbed quantum of ionizing radiation. The material under consideration should also have a relatively high average atomic number if used in gamma ray spectroscopy to increase the gamma ray interaction probability. High charge carrier mobilities and long charge carrier lifetimes are also needed to ensure efficient charge carrier extraction and minimal effects from position dependent charge collection.
CdZnTe (CZT), and particularly Cd.sub.1-x Zn.sub.x Te (where x is less than or equal 0.5), is a wide bandgap ternary II-VI compound semiconductor that because of its unique electronic properties is desirable for use in gamma-ray and x-ray spectrometers that operate at room temperature for nuclear radiation detection, spectroscopy and medical imaging applications. However, the performance of gamma-ray and x-ray spectrometers fabricated from CZT crystals is often limited by surface leakage currents. Surface leakage currents act as a source of noise that reduces the ability of these spectrometers to spectrally resolve the unique radiological emissions from a wide variety of radioactive isotopes. Thus, in order to improve the spectral resolution capability of devices based on CZT crystals it is desirable to decrease surface leakage currents and the attendant detrimental noise effects.
It is known in the art that for a semiconductor crystal to function effectively as a good detector material (i.e., minimizing surface leakage currents, thereby maximizing energy resolution) the crystal surfaces must be properly treated. Generally, this means chemical etching of the surfaces to eliminate undesirable surface features. Currently the method for surface treatment of CZT crystals is to chemically etch the crystal surfaces in a solution of bromine dissolved in methanol to provide a planar surface prior to attachment of electrical contacts. These solutions are used because they reliably produce surfaces on CZT crystals that are substantially planar and have a low surface leakage current. However, there is a need to reduce the surface leakage current in CZT crystals even further in order to improve spectral resolution. What is required is a method for surface treatment of CZT crystal that will eliminate or reduce surface leakage currents to a level that is presently unattainable using prior art methods.